1. Field of the Invention
This invention relates to the field of artificial dentures, and more particularly, to such dentures prepared from polyurethane elastomers and hard polymer components such as the hard acrylic resins and the hard epoxide resins.
2. Description of the Prior Art
It has been proposed to provide dentures with a soft layer in contact with the gums and other mouth parts to provide tissue relief. Such soft layers have been composed of acrylics, silicones, and similar rubber-like materials. But on aging, such soft layers tend to harden and give off undesirable odors. In addition, some decomposition of the polymer may also occur presumably due to an oxidation process as well as to pH fluctuations within the mouth. By way of overcoming these disadvantages, U.S. Pat. Nos. 4,024,636 and 4,080,412, both to Colpitts et al., and both incorporated by reference herein, describe dentures in which teeth are anchored in a gum member comprising a tooth-holding portion fabricated from a hard nonhydrophilic polyurethane elastomer having a hardness of not less than about Shore D40, and a mouth-engaging portion fabricated from a soft nonhydrophilic polyurethane elastomer having a hardness of not greater than about Shore A65 integrally and chemically bonded into a unitary mass. U.S. Pat. No. 4,024,637 to Colpitts which is also incorporated by reference herein describes a denture in which hard non-hydrophilic polyurethane elastomer teeth are imbedded in and chemically bonded to a soft non-hydrophilic polyurethane elastomer. Preferred non-hydrophilic elastomers are those formed by isocyanate-terminated prepolymers which are cross-linked or cured by mixing with a cross-linking agent and heating as required to effect curing. Isocyanate-terminated prepolymers suitable for preparing the hard non-hydrophilic polyurethane elastomers are prepared by the reaction of polyether diols or triols with aliphatic or cycloaliphatic or aralkyl di- or polyisocyanates in proportion to give free NCO groups. The prepolymers are then cured or cross-linked with a diol, polyol, an alkanolamine, a diamine or a tertiary amine containing polyol, or blends thereof. Advantageously, the diol or polyol is a polyether diol or polyol or a hydroxyl-terminated prepolymer.
By way of improving the resistance of polyurethane dentures to mechanical distortion or flex under the conditions prevailing in the mouth, U.S. Pat. No. 4,225,696 to Colpitts, et al., which additionally is incorporated by reference herein, substitutes the aforementioned aliphatic, cycloaliphatic or aralkyl di- or polyisocyanates with aromatic polyisocyanates in which the isocyanate groups are bonded directly to the aromatic nucleus, e.g., 2,4-tolylene diisocyanate (TDI), isomeric mixtures of TDI, 3,3'-tolidene 4,4'-diisocyanate (TODI), 3,3'-dimethyldiphenylmethane 4,4'-diisocyanate, diphenylmethane 4,4'-diisocyanate (MDI), mixtures of MDI and adducts of MDI, etc. The resulting polyurethane can be fabricated into the soft, mouth-engaging portion of a denture possessing a relatively hard polymer as the tooth-engaging portion thereof. The hard polymer can be a hard polyurethane prepared in accordance with any of the aforesaid Colpitts, et al., patents or it can be any of the hard polymers heretofore used in the making of dentures. As is well known, the acrylics, a class of relatively hard resins, have for many years been used in the manufacture of prosthodontic devices and would be prime candidates for preparing composite polyurethane/hard polymer dentures in accordance with the teachings of U.S. Pat. No. 4,225,696 to Colpitts, et al. However, as desirable an improvement as such composite dentures are, their polyurethane components which, as previously stated, are prepared from an aromatic isocyanate such as TDI, TODI or MDI, are relatively photosensitive and prone to degradation by actinic radiation. The probable explanation of this behavior is that when an isocyanate group reacts with water, it forms a urea group which, in the case of the aromatic isocyanates, is relatively chemically stable but light sensitive.
Accordingly, it is desirable to provide a denture which has a soft, mouth-engaging element to provide for the wearer's comfort and which at the same time is resistant to flex and photodegradation.